compiler: alignof [0]func() = 1

In the go protobuf code, a pattern is used to statically prevent
comparable structs by embedding:

```
type DoNotCompare [0]func()

type message struct {
  DoNotCompare
  data *uint32
}
```

Previously, sizezof(message{}) is 2 words large, but it only needs to be
1 byte.  Making it be 1 byte allows protobufs to compile slightly more
(though not all the way).

compiler/sizes.go

@@ -21,6 +21,12 @@ func (s *stdSizes) Alignof(T types.Type) int64 {
 	// of alignment of the elements and fields, respectively.
 	switch t := T.Underlying().(type) {
 	case *types.Array:
+		if t.Len() == 0 {
+			// 0-sized arrays, always have 0 size.
+			// And from the spec, should have an alignment of _at least_ 1
+			return 1
+		}
+
 		// spec: "For a variable x of array type: unsafe.Alignof(x)
 		// is the same as unsafe.Alignof(x[0]), but at least 1."
 		return s.Alignof(t.Elem())